In a cellular communications network, mobile devices (also known as User Equipment (UE) or mobile terminals, such as mobile telephones) communicate with remote servers or with other mobile devices via base stations. An LTE base station is also known as an ‘enhanced NodeB’ (eNB). When a mobile device attaches to the LTE network via a base station, a core network entity called Mobility Management Entity (MME) sets up a default Evolved Packet System (EPS) Bearer between the mobile device and a gateway in the core network. An EPS Bearer defines a transmission path through the network and assigns an IP address to the mobile device to be used by the mobile device to communicate with remote servers or other mobile devices. An EPS Bearer also has a set of data transmission characteristics, such as quality of service, data rate and flow control parameters, which are defined by the subscription associated with the mobile device and are established by the MME upon registration of the mobile device with the network.
The EPS Bearer is thus managed by the MME, which signals to the mobile device when it needs to activate, modify, or deactivate a particular EPS Bearer. Thus there are two connections between the mobile device and the communication network: one for the user data transmitted using the established EPS bearer (also known as the user plane or U-plane) and another one for managing the EPS Bearer itself (also known as the control plane or C-plane).
In their communication with each other, mobile devices and base stations use licensed radio frequencies, which are typically divided into frequency bands and/or time blocks. Depending on various criteria (such as the amount of data to be transmitted, radio technologies supported by the mobile device, expected quality of service, subscription settings, etc.), each base station is responsible for controlling the transmission timings, frequencies, transmission powers, modulations, etc. employed by the mobile devices attached to the base station. In order to minimise disruption to the service and to maximise utilisation of the available bandwidth, the base stations continuously adjust their own transmission power and also that of the mobile devices. Base stations also assign frequency bands and/or time slots to mobile devices, and also select and enforce the appropriate transmission technology to be used between the base stations and the attached mobile devices. By doing so, base stations also reduce or eliminate any harmful interference caused by mobile devices to each other or to the base stations.
In order to optimise utilisation of their bandwidth, LTE base stations receive periodic signal measurement reports from each served mobile device, which contain information about the perceived signal quality on a given frequency band used by (or being a candidate frequency band for) that mobile device. These signal measurement reports are then used by the base stations in their decision to allocate certain parts of their bandwidth to the served mobile devices and also to hand over mobile devices to other base stations (or other frequency bands/other radio access technologies (RATs)) when the signal quality does not meet the established criteria. The handing over of a mobile device might be necessary, for example, when the mobile device has moved away from the given base station, and also when an interference problem has arisen.
Current mobile devices typically support multiple radio technologies, not only LTE. The mobile devices might include, for example, transceivers and/or receivers operating in the Industrial, Scientific and Medical (ISM) radio bands, such as Bluetooth or Wi-Fi transceivers. Furthermore, mobile devices might also include positioning functionality and associated circuitry, for example Global Navigation Satellite System (GNSS) transceivers and/or receivers. Both ISM and GNSS (hereafter commonly referred to as non-LTE) radio technologies use frequency bands close to or partially overlapping with the LTE frequency bands. Some of these non-LTE frequency bands are licensed for a particular use (e.g. Global Positioning Systems (GPS) bands) or might be unlicensed bands and can be used by a number of radio technologies (such as Bluetooth and Wi-Fi standards using the same range of ISM frequency bands). The manner in which these non-LTE frequency bands are used are, therefore, not covered by the LTE standards and are not controlled by the LTE base stations. However, transmissions in the non-LTE frequency bands might, nevertheless, still cause undesired interference to (or suffer undesired interference resulting from) transmissions in the LTE bands, particularly in the overlapping or neighbouring frequency bands.
Such non-LTE radio technologies might be used by the mobile device itself or by other mobile devices in their vicinity and, although these radio technologies conform to the relevant standards (i.e. other than LTE), might still cause undesired interference to (or suffer interference from) the LTE transmission of these mobile devices. This is especially true when the end user is operating an ISM transceiver in parallel with the LTE transceiver, for example when the user is making a voice over IP (VoIP) call using a Bluetooth headset. It will be appreciated that in this case the LTE and ISM transmissions will interfere with each other as the LTE voice data received from the base station is relayed to the headset using the ISM transceiver implemented in the same mobile device. Thus any signal quality measurements performed by this mobile device before the VoIP call would not correspond to the actual signal quality perceived during the call. Furthermore, since an LTE base station can control only the mobile device's (and its own) LTE transmissions, any corrective measures made by the base station would inevitably fail to improve the interference perceived by the mobile device because of the concurrently operated ISM transceiver.
In another typical scenario, the LTE transceiver of the mobile device can cause interference to the GNSS receiver (e.g. a GPS receiver) making it difficult to obtain a current location of the mobile device. In this case, although there is no apparent disruption to the LTE signal (the signal quality measurements by the mobile device would indicate acceptable signal conditions), the LTE transmissions by the mobile device would be likely to render the GNSS functionality unusable because of the interference caused by the LTE transceiver to the GNSS receiver of the mobile device.
When interference such as this arises as a result of communication occurring concurrently in the same mobile device (for example, concurrent use of LTE and non-LTE radio technologies) the interference is sometimes referred to as ‘in-device coexistence (IDC) interference’ which causes an ‘in-device coexistence (IDC) situation’.
For mobile devices using the standard frequency bands simultaneously for LTE and ISM/GNSS radio communications, the typical in-device coexistence scenarios include:                LTE Band 40 radio transmitter causing interference to ISM radio receiver;        ISM radio transmitter causing interference to LTE Band 40 radio receiver;        LTE Band 7 radio transmitter causing interference to ISM radio receiver;        LTE Band 7/13/14 radio transmitter causing interference to GNSS radio receiver.        
However, other bands and/or other radio technologies might also experience interference due to LTE/ISM/GNSS transmissions.
In order to be able to alleviate the problems due to IDC interference, the mobile device indicates its IDC capability to its serving base station. If the received IDC capability of the mobile device indicates that the mobile device is able to do so, the serving base station configures the mobile device (by providing so-called ‘idc-config’ settings) to address IDC interference autonomously. Therefore, when the mobile device experiences interference due to an IDC situation, it can adjust its LTE and/or non-LTE transmissions in accordance with the ‘idc-config’ settings, thereby reducing or eliminating the experienced interference. In particular, the mobile device is allowed by the network to ‘deny’ (i.e. suspend or delay) its (already scheduled and hence expected) LTE transmissions up to a limit/rate specified in the ‘idc-config’ parameters. Essentially, this allows the mobile telephone to temporarily override the LTE scheduling decisions made by the network and to carry out ISM signalling and whilst its LTE transmissions are ‘autonomously’ suspended.
However, some IDC interference situations cannot be solved by the mobile device by itself, even if an ‘idc-config’ has been provided by the base station. In this case, the mobile device may need to send an IDC indication to the network (e.g. in an uplink RRC message) to inform the network about the IDC situation. To address such situations, 3GPP standards provide three techniques, using which the network (i.e. an LTE base station) is able to provide a solution when the mobile device cannot solve the problem by itself. The three techniques comprise: a TDM (Time Division Multiplexing) solution, an FDM (Frequency Division Multiplexing) solution, and a Power Control solution. It will be appreciated that ‘solution’ as used herein refers to control and/or configuration data that may be used by the mobile device to eliminate, or at least mitigate, the effects of the detected interference.
The TDM solution ensures that the transmission of a radio signal does not coincide with the reception of another radio signal. The FDM solution consists of choosing another serving frequency for the mobile device than the one suffering from interference. The Power Control solution aims to reduce radio transmission power to mitigate the effect of interference.
In order to benefit from these techniques, if the mobile device detects that an IDC situation is causing interference, it informs the base station (e.g. using Radio Resource Control (RRC) layer signalling) that an IDC situation has arisen and it provides assistance information (sometimes referred to as an ‘IDC assistance indication’) using which the base station is able to select the most appropriate technique to address the interference caused by the IDC situation. For example, the base station may select a different frequency for the mobile device indicating the IDC situation (FDM solution). Alternatively, the base station may reconfigure the transmission (e.g. apply discontinuous reception (DRX) and/or change its subframe pattern) (TDM solution) for that mobile device. The base station may also adjust its (or initiate adjustment of the mobile device's) transmission power (Power Control solution).
Further details of these techniques can be found in section 23.4 of the 3GPP TS 36.300 standards document (v.11.5.0). Details of the ‘idc-config’ settings can be found in the 3GPP TS 36.331 standards document (v.11.3.0). The contents of both documents are incorporated herein by reference.
However, the above solutions are not always applicable for the so-called small cell enhancement scenarios defined in 3GPP TR 36.932 (v.12.1.0). ‘Small cells’ in this context refer to the coverage areas of low-power nodes (for example Pico eNBs or Femto eNBs) that are being considered for LTE in order to support mobile traffic explosion, especially for indoor and outdoor hotspot deployments. A low-power node generally refers to a node that is operating a cell (‘small cell’) with a typical transmit power which is lower than typical transmit powers used in cells of macro nodes and base stations (‘macro cells’).
In particular, some of the small cell enhancement scenarios are based on a split control-plane/user-plane architecture (referred to as ‘C/U Split’), in which the mobile device is configured to maintain its control plane connection with the communication network via a macro cell (operating as a primary cell or ‘Pcell’) and at the same time maintain its user plane connection via one or more ‘small cells’ (operating as secondary cell or ‘Scell’) and thereby reducing the load in the macro cell. Effectively, in this case the mobile device is using two separate radio connections via two separate nodes (i.e. a macro base station and a low-power node), one for sending/receiving user data, and another one for controlling the mobile device's operations, such as mobility management, security control, authentication, setting up of communication bearers, etc.
In such situations, interference may occur on either radio connection. However, the mobile device is configured to receive its ‘idc-config’ settings (if any) from the macro base station, which settings are adapted to handle IDC situations arising in the macro cell (which carries the control plane only, in case of C/U-plane split) and thus cannot be used to tackle user plane interference experienced in the small cell. Even if the ‘idc-config’ would be adapted and/or applied to small cells (or both macro cells and small cells), due to the relatively large number of small cells, and their possibly differing operating characteristics (compared to the macro cell and also to other small cells), such ‘idc-config’ would not be capable of addressing all the different possible IDC situations.
Furthermore, the mobile device can only send an IDC assistance indication to the macro base station (via which it has a control plane connection), not the low-power node handling its user plane connection. However, given the relatively lower transmit power level used by the low-power node, user plane transmissions via the small cell might be more sensitive to interference than control plane transmission via the macro cell.
Even in the case when the mobile device experiences interference on the control plane and notifies this to the macro base station, any change in the macro base station's operation may cause unexpected interference for the user plane connection via the small cell.
There is therefore a need to improve the operation of the mobile device, the base station, and the low-power node in order to overcome or at least alleviate the above problems.